As described in the aforesaid patent application, the semiconductor laser diode has found application in a form which incorporates the diode, appropriate optics, and electronic driving circuitry within a unitary, modular package. A problem that has consistently been encountered in using the laser diode has been the need to achieve appropriate collimation, aiming, and frequently focusing of the beam, and methods for doing so have been quite varied.
For example, as described in the article "Header for Laser Diode," NASA Tech Briefs, April 1990, p. 16, a header package is described that incorporates a fairly extensive array of parts, including a diode housing, focus shim, diode retaining ring, collimator press-in retaining ring, course collimator, wave washer, both negative and positive fine-focus lenses, a pointing lens, and various other rings and retainers. Even so, it is asserted that the device still requires the alignment to be performed manually, in a process that can be both tedious and time consuming.
A somewhat simpler array of optical components is disclosed in U.S. Pat. No. 4,601,452 issued Jul. 22, 1986 to Rando, in which various mounting forks and diode holding blocks are employed to achieve side-to-side, up-and-down, and forward-and-back adjustments of the laser positioning, along with appropriate rotational adjustments of the laser beam relative to an aperture and an array of four lenses. Such a system, however, even though it may afford precise control of the resultant laser beam, is unduly complex and does not lend itself to final placement of the laser diode into a simple modular structure.
U.S. Pat. No. 4,541,689, issued Sept. 17, 1985 to Howard et al., discloses a laser diode pen that includes just three lenses and, although the method of aligning that system is not indicated, the specification does describe an additional friction wedge which serves to compensate for misalignments resulting from physical shock or thermal expansion. Even so, this disclosure does not recognize or address the fact that the axis of the laser diode radiation pattern will typically lie at some angle to the gross mechanical axis of the diode itself. Specifically, the beam pattern emitted by the laser diode itself will be found to be oriented in an arbitrary manner with respect to the TO-5 structure in which the diode is mounted, and it is only the orientation of that TO-5 structure that is subject to any control by a fabricator of devices that incorporate such a laser diode.
In patent application Ser. No. 07/524,152 noted above, an even simpler system for collimating and aiming the laser beam is described. As shown in FIG. 1 herein (corresponding to FIG. 3 of said application), those processes are carried out within the laser module itself, so that the beam parameters thus established are "locked into" the final laser module product in the course of its manufacture. More specifically, a laser module 10 is constructed within metal housing 12 having a cylindrical external sidewall 14, a front end wall 16 (consisting of axially-positioned output lens 18 and an adjustable annular mounting ring 20), and a rear wall 22. Laser diode 32 and its associated heat sink 34, cap 36, diode lens 38, and the timing and drive circuitry (which need not be further described) are incorporated onto printed circuit board (PCB) 30 and integrated within housing 12 to provide a laser beam transmitted along axis 24 when a DC voltage source (not shown) is connected thereto. In the manufacture of laser module 10, laser diode 32 is first mounted onto PCB 30 and the resultant structure is then placed within housing 12. The collimating and aiming processes are achieved very simply in this device through adjustment of the relative positioning of laser diode 32 and output lens 18.
Specifically,plano-convex output lens 18 is placed with its planar surface first into a concentric central opening of lens mounting ring 20, and mounting ring 20 is also sized and externally threaded to fit into front opening 46 of housing 12. Lens 18 has a cylindrical base portion which is sized to be received in the central opening of the mounting ring, and also a frontal flange that rests on the outer surface of the mounting ring. The lens is centered in the mounting ring by visual inspection and glue is then applied thereto. The complete lens structure (corresponding to front end wall 16) is mounted within housing 12 by screwing mounting ring 20 into front opening 46.
The distance between laser diode 32 and front opening 46 is fixed such that when front end wall 16 is screwed therein, the focal plane of laser diode 32 falls within the range of travel of lens 18 (positioned within mounting ring 20). In what has been termed a "focusing" step, mounting ring 20 is moved along the direction of axis 24 until the desired condition of "focus" is exhibited by external inspection of the laser beam. Such "focusing" refers not to an actual focal point, but instead to the duplication at the target of the desired sharp laser beam image. The collimation necessary to achieve that image is accomplished by the aforesaid placement of the planar surface of plano-convex lens 18 towards laser diode 32, and coincidentally by the placement of mounting ring 20 as just indicated.
Since the output beam of laser diode 32 as manufactured is not necessarily either co-directional or coaxial with its gross mechanical axis, it is also necessary to aim the exit beam of laser light so that the laser beam axis will coincide with axis 24. That is made possible by the fact that the external threads of mounting ring 20 are made loose enough relative to the internal threads of housing 12 about opening 46 to permit introduction of a sufficient amount of tilt and offset in positioning the lens to compensate for such variation in the orientation of the laser beam. To align the beam axis with axis 24, module housing 12 is placed in a clamp and then mounting ring 20, into which lens 18 has been placed and to which a small amount of glue has been applied, is placed therein and manipulated until such coincidence is achieved. The glue is then allowed to set. (So that proper aiming of the laser beam can be established, a target region is provided some distance down a darkened tunnel from the clamp that will hold housing 12, and the target "bullseye" is positioned so that its center coincides with axis 24 of a housing that has been so clamped. The laser beam is thus "aimed" when it is made to strike within an area defined by that "bullseye.")
Although the foregoing procedure is quite simple and involves the use of a minimum of optical components, nevertheless it suffers from the same defect as characterizes the procedure described in the NASA article cited earlier, namely, it still requires the alignment to be performed manually, in a manner that can be tedious and wasteful of time. Experience has shown, in fact, that in spite of the relative simplicity of the procedure as just described, this focusing and aiming process for a single module can require, on the average, about four minutes. (Part of that time period involves waiting for the glue to dry.) A second disadvantage of this procedure is that it depends upon subjective visual inspection by a human operator, and some range of error in the degree of focus (the quality of the beam image) and in the aiming accuracy must be expected. Therefore, there is a need for a rapid and accurate method of "focusing" (as above defined) and aiming a laser beam as emitted from a laser diode, given the fact that the axis of the laser beam as such cannot be expected to coincide with the gross mechanical axis of the laser diode structure.